1. Field of the Invention
The present invention relates to an elastic member for use in electrophotographic process or the like, and also to an image forming apparatus using the same.
2. Related Art
In electrophotographic apparatus such as electronic copy machine, laser beam printer, facsimile machine or the like, a developing process is performed by a developing device of the type wherein toner is frictionally electrified by rubbing a toner feed roller against a developing roller. Therefore, it is necessary for a toner feed roller to have a stable frictional ability with a developing roller and a high toner feeding ability to a frictional part. At the same time, it is also required for the toner feed roller to have the ability of scraping away excessive toner that remains on the developing roller without being used for development. From such viewpoint, an elastic material made of foamed rubber, polyurethane or the like, has been used up to present, for a toner feed roller of such a type.
However, there is an instance wherein a toner feed roller made of conventional foamed elastic material produces defects due to irregular pitch or irregular density on the developed image. Such defects are caused when the toner is insufficiently or unevenly supplied by the toner feed roller, or when the toner is insufficiently or unevenly scraped away, and may produce indistinct image due to reduction in density of the image, for example.
Consequently, there is a demand for a foamed elastic member that can be suitably used in an electrophotographic apparatus, particularly as a toner feed roller, for achieving an image free from defects such as pitch irregularity, density irregularity or the like, or free from reduced density.
It is a primary object of the present invention to provide a foamed elastic member suitable for use in an electrophotographic apparatus, particularly as a toner feed roller, capable of providing an image free from such defect as pitch irregularity, density irregularity or the like, or free from reduction in density.
In order to achieve the object, the inventors conducted thorough investigations on a toner feed roller that has produced defective image as described above, and found the following. That is to say, when a high-density image such as a black solid image is printed immediately after a low-density image such as a white solid image has been printed, an irregularity in density occurs such that the leading end of the printed black solid image is higher in density than the trailing end of the printed image. It is noted that, in the case of white solid printing, the amount of toner carried from a developing roller to a photosensitive member is small and the amount of the toner remaining on the developing roller is large, with the result that the toner is liable to be insufficiently scraped away by a toner feed roller and the amount of residual toner on the developing roller is gradually increased. Therefore, the irregularity in density is caused when a black solid image is printed immediately after a white solid image is printed, resulting in a higher density of an image printed by one to two rotations of the developing roller as compared to the density of the trailing end of the printed image, or when the amount of toner supplied to the developing roller by the third to fourth rotations of the developing roller corresponding to the hind part of the printed image is insufficient as compared to the toner amount supplied by the first to second rotations of the developing roller.
The present invention has been achieved based on a conception that, when a foamed elastic material obtained by foaming rubber, polyurethane or the like is used as a toner feed roller, in order to exhibit excellent performance of feeding toner and performance of scraping toner away to thereby form an image free from defect, it is important to properly determine physical property values at the surface of the foamed elastic material and/or to properly determine the shape and arrangement of the foam cells opening at the surface of the foamed elastic material.
According to a first aspect of the present invention, there is provided a foamed elastic member for use in an electrophotographic process or the like, comprising an elastic member made of polyurethane foam molded by an in-mold foaming process, and having a plurality of openings regularly arranged on the surface of the polyurethane foam, each of said openings having a basic shape approximate to a circle, ellipse, racetrack, triangle, tetragon or hexagon.
Since the polyurethane-foam elastic member as described has properly determined shape and arrangement of the foam cells that are opened on its surface, it particularly advantageously exhibits excellent toner feeding performance and toner scraping performance when used as a toner feed roller in a developing device for electrically charging toner from a toner storage portion, feeding the toner to the surface of a photosensitive member and forming a toner image corresponding to an electrostatic latent image on the surface of the photosensitive member.
The inventors conducted further investigations and found that it is possible to form an image free from defect by improving the toner feeding performance and the toner scraping-away performance of a toner feed roller when optimization is made of such parameters as the average opening diameter of the foam cells opened at the surface of a foamed elastic member forming the toner feed roller (i.e., xe2x80x9caverage cell opening diameterxe2x80x9d), the ratio of the area of openings of the foam cells to the surface area of the foamed elastic member (i.e., xe2x80x9ccell opening ratioxe2x80x9d), the compression spring constant and the restoring ratio of elasticity after compression of a skin layer in the vicinity of the surface of the foamed elastic member, as well as the coefficient of friction of the surface of the foamed elastic member, etc.
According to a second aspect of the present invention, there is provided a polyurethane-foam elastic member molded by an in-mold foaming process, and satisfying the following conditions:
(A) the average opening diameter at the surface of the foamed elastic member is 50 to 300 xcexcm,
(B) the ratio of the total area of the openings to the surface area of the foamed elastic member is 50 to 80%,
(C) the compression spring constant of the foamed elastic member is 0.25 to 1.5 N/mm,
(D) the restoring ratio of elasticity after compression of the foamed elastic member is 60% or more, and
(E) the coefficient of friction of the surface of the foamed elastic member is 0.4 to 3.0.
Since the polyurethane-foam elastic member as described above has properly determined physical property values at its surface are, it particularly advantageously exhibits excellent toner feeding performance and toner scraping performance when used as a toner feed roller in a developing device for electrically charging toner from a toner storage portion, feeding the toner to the surface of a photosensitive member and forming a toner image corresponding to an electrostatic latent image on the surface of the photosensitive member.